Microelectromechanical system (MEMS) gyroscopic devices are utilized in a variety of applications for sensing inertial motion in one or more dimensions. Such devices are particularly useful in applications demanding a high degree of reliability and accuracy where it may be necessary to measure and/or detect small changes in motion or acceleration, or where size and/or weight are important design considerations. In the design of navigational and communications systems, for example, such devices are useful in measuring and/or detecting slight variations in linear and rotational motion of an object traveling through space. Because such devices can be manufactured using batch semiconductor fabrication techniques (e.g. photolithography), greater tolerances and reliability can be achieved in comparison to more traditional fabrication techniques.
The design of MEMS-type gyroscopes varies greatly depending on their particular purpose. Rate gyroscopes, for example, are often used to determine the rate of rotation of a moving object by generating and measuring Coriolis forces. In a vibratory-type rate gyroscope, for example, a drive system including one or more proof masses can be configured to oscillate back and forth relative to a motor pickoff comb in a drive plane orthogonal to the input axis, or “rate axis,” in which motion is to be determined. The proof masses may each include a number of interdigitated comb fingers configured to move relative to each other when electrostatically charged with a time-varying signal from a drive voltage source. A number of suspension springs or other flexural elements are typically used to constrain motion of each proof mass in a particular direction above an underlying support substrate.
A sense electrode or other sensing means disposed on the substrate adjacent to and parallel with each proof mass can be charged with a sense bias voltage. As each proof mass moves back and forth above the substrate, the Coriolis force resulting from conservation of momentum of the moving body as it rotates about the input axis causes the spacing between each proof mass and sense electrode to vary, resulting in a concomitant change in capacitance. By measuring the capacitance between the proof mass and sense electrodes in this manner, a measure of the rotational motion and/or acceleration of the moving body can be ascertained.
A significant source of errors in many MEMS-type gyroscopes is due to quadrature motion (i.e. motion out-of-plane) of the proof mass as it oscillates back and forth above the sense electrode. Such quadrature may result, for example, from imperfections in the profile of the comb fingers and suspension springs used in the drive system, and from other imperfections created during the manufacturing process. When present, such quadrature motion produces a large sense signal, affecting the ability of the gyroscope to accurately discern subtle variations in the rate signal. Such quadrature can also interfere with the drive electronics used to drive the proof masses, in some cases requiring additional error-correction circuitry. As a result of these irregularities in the drive system, the output sense signal may contain both the desired rate signal as well as an undesired quadrature signal, diminishing the ability of the gyroscope to effectively measure the rate motion.
In some cases, the temperature within the environment surrounding the gyroscope may also affect the ability of the gyroscope to sense subtle variations in the rate signal. Generally speaking, as the temperature of the gyroscope increases, it becomes more difficult to drive the proof masses, thus increasing the magnitude of the voltage required to drive the proof masses. In some designs, the presence of heat may cause the proof masses to rise or lift above the substrate surface in the direction of the sense axis, introducing an undesired thermal component into the sense signal.
To compensate for quadrature, vibration, and other irregularities in the drive system, many prior-art designs have focused on methods and techniques for suppressing the undesired quadrature signal using complicated error-correction techniques in the drive system and/or sensing circuitry. In certain prior-art designs, for example, a frequency translation circuit is employed to suppress components of the drive signal to the sensor output signal at a predetermined frequency range so that the drive signal can be disassociated with the sense signal using conventional filtering techniques. In other prior-art designs, quadrature error is reduced via a synchronous demodulation process, wherein the desired rate signal is maintained 90° out-of-phase with the quadrature signal. While such methods have been employed to compensate for the undesired quadrature signal, such techniques do not eliminate the undesired quadrature motion in the drive system. As such, many prior-art gyroscopes are unable to accurately detect and measure subtle changes in motion effectively.